1. Field of the Invention
The present invention relates to the structure of the scroll of a variable-throat exhaust turbocharger equipped with a nozzle throat area varying mechanism (hereafter referred to as a variable-throat mechanism) and a method of manufacturing the turbocharger. The turbocharger is used for an internal combustion engine, the exhaust gas of the engine being introduced into the scroll chamber. The variable-throat mechanism is composed to vary the blade angle of a plurality of nozzle vanes disposed in an annular flow passage through which the exhaust gas enters the turbine rotor of the turbocharger to exert force to rotate the turbine rotor.
2. Description of the Related Art
In the field of relatively small exhaust turbochargers for internal combustion engines for vehicles, a radial flow type variable-throat exhaust turbocharger equipped with a variable-throat mechanism is widely used. Such a turbocharger is disclosed for example in JP2001-207858A, in which engine exhaust gas is introduced to the scroll chamber formed in the turbine casing to allow it to flow through an annular flow passage where a plurality of nozzle vanes are disposed so that the blade angle thereof can be varied, and the exhaust gas exerts forces to rotate the turbine rotor.
In the turbocharger of JP2001-207858A, the engine exhaust gas enters the scroll chamber of the turbine casing and flows through the space between the nozzle vanes to enter the exhaust turbine while flowing along the convolute passage of the scroll chamber in the turbine casing. The exhaust gas enters the turbine from the outer periphery side thereof to flow radially inwards, expands therein to transmit expansion work thereto and flows out from the turbine in the axial direction to be exhausted outside.
Controlling of the exhaust gas flow rate of the turbocharger is performed by determining a blade angle of the nozzle vanes so that the exhaust gas flow rate becomes a desired flow rate. Linear displacement of an actuator to correspond with the desired blade angle converts the displacement to rotation of the nozzle shaft by the medium of a drive ring, drive pins, and lever plates so that the nozzle vanes are rotated to be at the desired angle, and thus the blade angle is varied by the actuator.
Another example of a radial flow type variable-throat-exhaust turbocharger equipped with a variable-throat mechanism is disclosed in JP2004-132367A. A nozzle assembly of a variable-throat mechanism small in size, simple in construction, and easy to assemble.
In JP2004-138005A is disclosed a member, which contacts to another member and slides on it, and a member which faces another member with a very small clearance and has relative motion arising between the members, which are dipped in a solution of xylene or the like and micro carbon powder to have the surfaces of the members coated with dry carbon films. By this, the friction of sliding is reduced and an appropriate small clearance is always maintained.
In a radial flow type variable-throat exhaust turbocharger equipped with a variable-throat mechanism, cross sectional area A of the convolute passage of the scroll chamber and a scroll radius R (distance from the center of rotation of the turbine rotor to the center of cross sectional area A of the convolute passage) are important design factors. This A/R ratio is an important factor for determining aerodynamic performance and flow characteristic of the turbine. When it is intended to increase the A/R ratio, it is necessary to increase cross sectional area A or decrease scroll radius R.
With the exhaust turbine disclosed in JP2001-207858A, when it is intended to increase cross sectional area A of the convolute passage, the passage must be widened radially and axially, which results in an increased size of the turbine casing. Particularly, when the scroll chamber is widened radially outwardly, scroll radius R increases and the A/R ratio does not increase much.
When intending to decrease scroll radius R, it is needed to displace the scroll chamber radially inwardly, but it is difficult because there is, in the radially inner side of the scroll chamber, an annular flow passage in which the nozzle vanes are located.
Scroll radius R can be decreased by forming the scroll chamber more planar, but the scroll chamber must be extended in the axial direction and the turbine casing becomes rather large.
It is proposed to widen the scroll chamber largely axially inwardly to increase the A/R ratio, but in that case, manufacturing of the core for forming the scroll chamber when casting is more difficult and productivity is poor.
In JP2004-132367A is shown, in FIG. 1, a variable-throat exhaust turbocharger in which the annular disk/connecting ring 29 composes a part of the scroll chamber. However, the periphery of the disk/connecting ring 29 defines part of inner surface the scroll chamber and the cross sectional area of scroll passage area can not be increased without increasing the outer diameter or width of the turbine casing.
The invention disclosed in JP2004-138005A is very effective for reducing friction between sliding members or always maintaining an appropriate small clearance between members facing to each other with a small clearance. However, it is inevitable that the manufacturing cost increases due to the additional processing.